package com.bo.day20231123;

import java.util.Arrays;

/**
 * 逆序对问题：https://leetcode.cn/problems/shu-zu-zhong-de-ni-xu-dui-lcof/description/
 * 一个数组中,当左边的数和右边的数存在一个降序,则存在一个逆序对
 * 例如：arr={3,0,8,1,6,7,0,1}
 * 3：(3,0),(3,1),(3,0),(3,1) 4个
 * 0：0个
 * 8：(8,1),(8,6),(8,7),(8,0),(8,1) 5个
 * 1：(1,0) 1个
 * 6：(6,0),(6,1) 2个
 * 7：(7,0),(7,1) 2个
 * 0：0个
 * 1：0个
 * arr中存在4+0+5+1+2+2+0+0=14个逆序对
 *
 * @Author: gpb
 * @Date: 2023/11/27 11:14
 * @Description:
 */
public class ReverseOrderPair {

    public static void main (String[] args) {
        int[] arr = {3, 0, 8, 1, 6, 7, 0, 1};
        System.out.println(reversePairs(arr));
        System.out.println(Arrays.toString(arr));
        int testTime = 500000;
        int maxSize = 100;
        int maxValue = 100;
        System.out.println("测试开始");
        for (int i = 0; i < testTime; i++) {
            int[] arr1 = generateRandomArray(maxSize, maxValue);
            int[] arr2 = copyArray(arr1);
            if (reversePairs(arr1) != comparator(arr2)) {
                System.out.println("Oops!");
                printArray(arr1);
                printArray(arr2);
                break;
            }
        }
        System.out.println("测试结束");
    }


    public static int reversePairs (int[] record) {
        if (record == null || record.length < 2) {
            return 0;
        }
        return process(record, 0, record.length - 1);
    }

    public static int process (int[] arr, int l, int r) {
        if (l == r) {
            return 0;
        }
        int mid = l + ((r - l) >> 1);
        // 左部分 + 右部分
        return process(arr, l, mid) + process(arr, mid + 1, r) + merge(arr, l, mid, r);
    }

    public static int merge (int[] arr, int l, int mid, int r) {
        // 辅助数组
        int[] help = new int[r - l + 1];
        // 辅助数组下标
        int i = 0;
        int p1 = l;
        int p2 = mid + 1;
        int ans = 0;
        while (p1 <= mid && p2 <= r) {
            ans += arr[p1] > arr[p2] ? (r - p2 + 1) * 1 : 0;
            help[i++] = arr[p1] > arr[p2] ? arr[p1++] : arr[p2++];
        }
        while (p1 <= mid) {
            help[i++] = arr[p1++];
        }
        while (p2 <= r) {
            help[i++] = arr[p2++];
        }
        for (i = 0; i < help.length; i++) {
            arr[l + i] = help[i];
        }
        return ans;
    }

    // for test
    public static int comparator (int[] arr) {
        int ans = 0;
        for (int i = 0; i < arr.length; i++) {
            for (int j = i + 1; j < arr.length; j++) {
                if (arr[i] > arr[j]) {
                    ans++;
                }
            }
        }
        return ans;
    }

    // for test
    public static int[] generateRandomArray (int maxSize, int maxValue) {
        int[] arr = new int[(int) ((maxSize + 1) * Math.random())];
        for (int i = 0; i < arr.length; i++) {
            arr[i] = (int) ((maxValue + 1) * Math.random()) - (int) (maxValue * Math.random());
        }
        return arr;
    }

    // for test
    public static int[] copyArray (int[] arr) {
        if (arr == null) {
            return null;
        }
        int[] res = new int[arr.length];
        for (int i = 0; i < arr.length; i++) {
            res[i] = arr[i];
        }
        return res;
    }

    // for test
    public static boolean isEqual (int[] arr1, int[] arr2) {
        if ((arr1 == null && arr2 != null) || (arr1 != null && arr2 == null)) {
            return false;
        }
        if (arr1 == null && arr2 == null) {
            return true;
        }
        if (arr1.length != arr2.length) {
            return false;
        }
        for (int i = 0; i < arr1.length; i++) {
            if (arr1[i] != arr2[i]) {
                return false;
            }
        }
        return true;
    }

    // for test
    public static void printArray (int[] arr) {
        if (arr == null) {
            return;
        }
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
        System.out.println();
    }

}
